In the development of solid oxide fuel cells (SOFCs), there is a need to produce low cost electrically-connecting components such as interconnects typically used between the fuel cells in stacked or bundled cells. In planar stack designs, flat plate interconnect components interleave with the fuel cells, and function as a fuel/air gas separator and electrical connection between cells. In tubular designs, wires, foams, or meshes are used to provide electrical connection between the cells in a bundle.
There is need for a highly conductive electrical component (plate, foam, mesh, etc.) that is robust to long-term service in an oxidizing environment. Purely ceramic materials have been identified as interconnects and current collectors in some stack designs, but such materials are extremely expensive to produce and have poor mechanical stability. Refractory precious metals that resist oxidation can also be used, but are typically cost-prohibitive for commercial products. Powder-metallurgy derived alloys exhibit many of the requisite properties but also suffer from chromium volatilization. The most economically-attractive solutions are often stainless steel alloys which can be produced at low cost in a range of forms.
Several alloys have been proposed, including Ni—Cr containing superalloys, which form conductive chromium-containing spinel coatings. The high nickel content of these alloys makes them cost-prohibitive for commercial applications. Ferritic stainless steel alloys containing Cr at 18% or more are more cost-effective choices. These alloys develop continuous protective scales of chromium oxide or chromium-containing spinel upon oxidation (“native scales”). Inexpensive 430 stainless steel forms Cr2O3 when oxidized. However, this scale grows and sheds continuously, resulting in a series of spalling events that destroys connectivity between cells.
Crofer 22APU and AL441-HP are ferritic stainless steel alloys having improved corrosion resistance, forming a conductive (Mn,Cr)3O4 spinel scale during oxidation, which grows slowly and extends service life. However, the (Mn,Cr)3O4 spinel has low conductivity and does not prevent the subscale growth of Cr2O3. Furthermore, the volatilization of CrO(OH) over Cr-containing oxides is finite, and it poisons SOFC cathode performance and reduces cell performance. Thus, a path to thin, high conductivity, low Cr-content oxide scales is often desired for planar SOFCs intended for long term operation.
In light of the foregoing, various oxide coatings have been developed to combat corrosion in SOFC stacks and the like. Such coatings can enhance the electrical connection between the metal and the cathode contact paste, and can also slow the growth of low-conductivity chromium oxide and chromium spinel layers. Further, these layers can retard the volatilization of Cr from the alloy surface.
A range of materials have been suggested, including transition metal spinels, which can be inexpensively synthesized and deposited. Spinel coatings are applied by the aerosol spray deposition of slurry consisting of powder of the targeted composition or a mixture of precursor powders that are reaction-sintered to produce an oxide scale of the targeted composition.
Unfortunately, the relatively low bulk conductivity (typically, 60 S/cm or lower) of spinels at SOFC operating temperatures increases stack resistance. To achieve the stoichiometry required to maximize conductivity, spinel coatings are first fired in a reducing atmosphere to form a cermet and then re-oxidized. This increases process complexity and cost. State-of-the-art spinel coatings applied in this manner typically demonstrate area specific resistance values of 5-20 mΩ-cm2.
Alternatively to spinel coatings are perovskite oxide coatings, which, in some instances, offer higher electrical conductivity (100-150 S/cm) with reasonable thermal expansion match to terrific stainless steel. Coatings based on alkaline earth doped perovskites such as (La,Sr)MnO3 and (La,Sr)CrO3, similar in composition to purely ceramic interconnect materials, have been evaluated. However, the refractory nature of these materials has required the use of specialized deposition technologies including sputtering, PVD and pulsed laser deposition. These processes produce dense, uniform coatings, but they require expensive, complex capital equipment for manufacturing. These coatings also require very high processing temperatures and/or exotic applications processes. In addition, they exhibit p-type conductivity rather than n-type, and furthermore exhibit desirable conductivity (>50 S/cm) only at the high end of the SOFC operating range (T=800 C or higher).
Mixed-conducting alkaline earth doped perovskite coatings such as (La,Sr)FeO3 and (La,Sr)CoO3 also have been applied to metals, presenting low initial ASR resistance in testing. However, the resistance of such coated materials increases rapidly, due to native scale growth (oxidation of the underlying alloy from high oxygen flux through the coating) and Cr diffusion into the perovskite coating that forms low-conductivity solid solutions. Furthermore, as Cr diffuses to the surface of the coating, Cr volatilization can occur.
Spinel coatings (oxides typically of the formula AB2O4, where A is a 2+ valent cation and B is a 3+ valent cation, possessing the crystal structure of the mineral spinel MgAl2O4), most commonly of the transition metal series of elements (particularly Mn, Ni, Co, Cu, and Fe) have also been evaluated as coating materials for this application. These coatings are typically applied by a redox firing method. However, these coatings have overall lower conductivity values and imperfect thermal expansion match with the underlying stainless steel.
Thus, it is clear that a need exists for a coating for stainless steel and other metallic substrates used, for example, in SOFCs, wherein the coating provides one or more benefits. Such benefits may include, for example, improved corrosion resistance, improved electrical contact and/or reduced chromium volatility. While a variety of oxide coatings have been made and used for such purpose, it is believed that no one prior to the inventors has made or used an invention as described herein.